1. Field of the Invention .
The invention relates to a temperature sensor using optic fibers and to their properties whereby they have birefringence which varies as a function of temperature.
2. Description of the Prior Art
A linear polarization preserving monomode fiber is obtained by creating a high degree of elasto-optical birefringence, using a zone that sets up intrinsic stresses, in a monomode fiber A standard example of a fiber of this type has the following structure: two very highly doped bars are placed on either side of the core. During the fiber-drawing process, the different glasses are first viscous and then get solidified. In cooling down to ambient temperature, the doped bars, which have a thermal expansion coefficient far greater than that of the rest of the structure, get contracted and therefore place the region of the core under elongation stress By elasto-optical effect, this stress creates birefringence.
This birefringence is often defined by the beat length L.sub.B, namely the length at the end of which the polarizations along the two neutral and orthogonal axes of the fiber have become phase-shifted by 2.pi.rad. This length is typically of the order of 1 to 5 mm. The origin of this stress is, therefore, the cooling of the non-homogenous structure and depends on the temperature. This dependence is of the order of 10.sup.-3 /.degree. C.